1. Field of the Invention
The present invention relates to substrate handling systems and, in particular, relates to non-contact end effectors for transferring substrates.
2. Description of the Related Art
Integrated circuits which comprise many semiconductor devices, such as transistors, diodes, and resistors, are often fabricated on a thin slice of semiconductor material, otherwise known as a substrate. These semiconductor devices are often formed in the substrate in an epitaxial process or a doping process that involves positioning the substrate in a plurality of high temperature chambers where exposed portions of the substrate are exposed to high temperature doping gases which results in doped layers being selectively formed in the device. Consequently, when forming such integrated circuits, it is often necessary to remove the substrate from one high temperature chamber having a first doping or epitaxial species and reposition the hot substrate having a temperature as high as 900.degree. degrees Celsius to another high temperature chamber having a different doping or epitaxial species. However, since the substrate is extremely brittle and vulnerable to particulate contamination, great care must be taken so as to avoid physically damaging the substrate while it is being transported, especially when the substrate is in a heated state.
To avoid damaging the substrate during the transport process, various well known semiconductor substrate end effectors have been developed. In particular, one class of end effectors, known as Bernoulli wands, are used for transporting very hot substrates. The advantage provided by the Bernoulli wand is that the hot substrate generally does not contact the pickup wand, except perhaps at one or more small locators positioned on the underside of the wand. Such a Bernoulli wand is shown in U.S. Pat. No. 5,080,549 to Goodwin, et al.
In particular, when positioned above the substrate, a plurality of gas jets that emanate from a lower surface of the Bernoulli wand create an airflow pattern above the substrate so as to cause the atmospheric pressure immediately above the substrate to be less than the atmospheric pressure immediately below the substrate. Consequently, the pressure imbalance causes the substrate to experience an upward "lift" force. Moreover, as the substrate is drawn upward toward the wand, the same jets that produce the lift force produce an increasingly larger repulsive force that prevents the upper surface of the substrate from substantially contacting the lower surface of the Bernoulli wand. Thus, the substrate is forced into a state of levitation at a vertical equilibrium position below the lower surface of the Bernoulli wand.
To engage the substrate in a horizontal manner, the plurality of gas jets that emanate from the lower surface of the Bernoulli wand are adapted to have a lateral bias. Thus, the substrate experiences a lateral force that results in the substrate being pinned against the small locators of the Bernoulli wand that extend below the lower surface of the Bernoulli wand. Consequently, although contact between the upper surface of the substrate and the lower surface of the Bernoulli wand is avoided, a portion of a side surface of the substrate often contacts the locators of the Bernoulli wand.
Although Bernoulli wands known in the art are able to manipulate very hot substrates, they require a relatively large gas flow rate that can exceed 90 liters per minute. Since typical gas supply systems may include undesirable particles in the flow of gas, it is possible for such gas flows to adversely affect the substrate. In addition, the high gas flow will agitate otherwise settled particles that exist within system. Although the effects of particle contamination can sometimes be reduced by utilizing pressurized gas systems having relatively low levels of particle contamination, such gas systems usually require a significant financial expense.
Another disadvantage of some Bernoulli wands is that they are relatively expensive to manufacture. In particular, the typical Bernoulli wand is comprised of a pair of quartz plates that are fused together in a relatively expensive process to form a composite structure having an integrated network of gas channels formed between the quartz plates. Furthermore, since the gas flow pattern produced by the Bernoulli wand is relatively complicated, the network of gas channels formed within the quartz plates must be formed in an elaborate manner, thereby increasing the manufacturing costs even further.
A further disadvantage of using Bernoulli wands is that the side surface of the substrate is contacted by the Bernoulli wand. In particular, such contact can cause the substrate to experience thermal shock at the region of contact, which can possibly damage a significant portion of the substrate. Furthermore, such contact can enable contaminants inadvertently deposited on the locators of the Bernoulli wand to be transferred to the substrate, thereby limiting the yield of the semiconductor processing system.
As an alternative to Bernoulli wands, spatula-type end effectors are sometimes used in the semiconductor processing industry. In particular, spatula end effectors are formed with a flat upper surface that is adapted to engage a lower surface of the substrate. Specifically, the upper surface of the spatula is positioned below the lower surface of the substrate so that three or more pins that extend above the upper surface of the substrate spatula are allowed to contact the lower surface of the substrate so as to support the substrate. Thus, frictional engagement between the pins of the spatula and the lower surface of the substrate help to prevent the substrate from sliding with respect to the spatula during movement of the spatula.
However, since typical substrate spatulas directly contact the lower surface of the substrate, the initially hot substrate must be allowed to cool before it can be engaged by the substrate spatula so as to avoid unacceptable damage to the substrate. Since the initial temperature of the substrate can be as high as 1200.degree. degrees Celsius and the acceptable handling temperature is around 500-600.degree. degrees Celsius, the cooling period can be considerable. Thus, prior art semiconductor processing systems that utilize typical substrate spatulas often suffer from limited production capacity due to the foregoing cooling delays.
From the foregoing, it will therefore be appreciated that there is a need for a relatively inexpensive end effector that is capable of supporting and moving a very hot semiconductor substrate or other substrate in a non-contaminating manner so as to improve the product quality and production capacity of known substrate processing systems. To this end, there is a need for an improved end effector that utilizes a greatly reduced flow of gas to support and move the substrate in a substantially non-contacting manner.